measurements for venting

I think i finally have resolved the question of my needs for venting my new roof. Thanks to those of you who helped getting a complete newbie even to this point! I am figuring that i have 1800 sf of attic space and approximately 11sf of sofits. There might actually be more soffit because 5 of them are under the eve of a flat roof; I am not sure if only the flat roof is vented or if there is a flow through the flat roof and into my pitched roof. The flat roofed porch is original to the house so I imagine it is possible. Based on the square footage of the attic i should need 29 linear feet of ridge vent, or a little more depending on the type. I only have, at most, 25 feet of ridge space available --would have been nice in terms of the balance with the soffits, but it seems this is not enough ridge space-- and no combination of ridge vent and other roof vent seems to be recommended. I guess it is necessary to close off the passive vents I now have and use mechanical ventilation.

But the one company whose products i began to read about with electric or solar vents figures its products based on 150 NFVAs while OC figures the linear footage for ridge vents based upon 300 NFVAs and in a rare case 150. I don't think i am the rare case. Here are the questions:

Do the different kinds of ventilation have different requirements in terms of NFVAs or is the company selling the fans being extremely conservative. Based on the fan company's standards I should get two of their fans. These would cover 2400 square feet. (can ever over-vent the attic?)

How do you figure your needs for soffit vents with mechanical vents-- solar or electric fans? Do they just add to the inflow and circulation and does the amount of vent space at the eaves matter; do i have to increase or decrease the number of louvered soffit vents that I have.?